Apparatus for testing the aerodynamic balance of airscrews



Feb. 12, 1946. F. B. HALFORD APPARATUS FOR TESTING THE AERODYNAMICBALANCE OF AIRSCREWS Filed Sept. 4. 1942 3 Sheets-Sheet 1 Feb. 12, 1946.-0 D 2,394,766

APPARATUS FOR TESTING THE AERODYNAMIC BALANCE OF AIRSCREWS Filed Sept.4, 1942 s Sheets-Sheet z F/aJ,

Ill",

1946- 'F. B.-HALFORD I APPARATUS FOR TESTING THE AERODYNAMIC BALANCE OFAIRSCREWS I I Fi-led Sept. 4, 1942 3 Sheets-Sheet 3 the "aircraft.

Patented Feb. 12, 1946 APPARATUS FOR TESTING I'TH'E-"AIERODY- NAMICBALANCE OFI-AIRSCREWS Frank-Bernard Halford, HatfieliluEngtland, as-

signor to The De HavillandAircraff Company Limited, Hatfield Aerodrome,England 'Applicationlseptem'ber 4, 1942,. SerialfNo. 451,351

, In Great Britain Augustx21, 1941 '3' Claims.

This invention relates to apparatus for testin the aerodynamic balanceof 'airscrews.

In modern high-performance aircraft it may sometimes occur that despitethe fact that th'e airscr'evv :may be effectively mass-balanced :bothstatically and dynamically serious vibration may arise as a resultofaerodynamic unbalance of the individual airscrew blades due to minuteinequalities in their effective pitch, producin differences in theirthrusts and hence'a coupieon Since the plane of action ofthe couplerotates with the airscrew its effect is the same -as that 'of dynamicmass unbalance of :an airscrew which i statically balanced.

It is'an object. of the invention to zprovide' app'aratus for testingthe aerodynamic balance of "airscrewblades, it being understood thatthe'data afforded by the test may be an indlcation of the degree ofbalance, or lack of it; the setting :at which the best condition ofbalance is achieved for an individual airscrew blade,-or' for the bladesof an airscrew embodying a plurality; or the blade relationship ofanygiven blade or airscrew toa masterblade.

The present invention consists in apparatus for testing the aerodynamicbalance of "airscrew blades and includes an airscrew hub adaptedandarranged for the reception of one or more .production blades to beaerodynamically balanced .against amaster blade or -aplurality of masterblades; means for driving the airscrew hub, the assembly of drivingmeansand hub being mounted with freedom to oscillate about-anaxistransverse to the axis of the hub; means formeasuring or indicating theoscillations imparted to the said assembly in operation thereof; -meanstor adjusting the pitch setting of at least one of the blades; and meansreadily operable to ensure secure-engagement and later disengagementrelative tothehub of a-blade to be tested. 'The 05011- lations ofthe-frame preferably occur in opposition to resilient means a'iiorded bysprings or theilike. The pitch-changing means may oper ate in anyconvenient manner, and preferably are capable of being actuated to varythe pitch during actualoperation or" the apparatus. Thep'itchchangingmeans maybe connected to the master blade and/or with the productionblade to be tested. If the pitch-changing means is '(or are) connectedwith the master blade, the pitchchange control mechanism is less likelyto be deranged by continual insertion and removal of the productionblades to be tested against "the master; While, furthermore, the shankof the m'aster'bla'de can then be di'iierent'fromthe shank of theproduc'titnibladein order to achieve con siderable simplification of thepitch-changing mechanism.

In ordertthat it'may be clearly understood and readily carried intoeffect, the invention is hereinafter described with reference to theaccompanying -'diagrammatie'drawings, of which:

Figure 1 is a fragmentary side elevation .partly insecticnfillustratingairscrew blade balance testing apparatusaccording to the invention;

Figure 2 is a p'lan view corresponding to Figure 1:;

Figure 3 is a sectional elevation taken longitudinailly through thevhub'drive and'pitch-change mechanism of the apparatus according to thepresent invention the Kline 0f section being indicated at III-4 11inFigure-4;

Figure i is a section on the line FIV-IV of Figure 5;

"-Figureb is a-"secti'on on the line V- V of Figureal;

Figure -6"'illustrate's "one method of blade attachment-f the partsbeing shown in Figure 6 in the fully-engaged position; while Figure "7-illustrates' the blade-attachment means in a position of initialengagement.

Rferring first to Figures-I and -2-o'f the drawings, the apparatus-ismounted on a rigid stand "or-base I 0 whichsupports the electric motorll by meansoi the links [3 and I4, pivotally mounte'dat eac'h'en'd. Themotor I I drives the vairscrew 'hub 15 through the reductionge'ar-boxi6, The 'hubfis arranged to mount a master blade, represented by thereference letter "M; and a production blade, represented by thereference letter P; and-in accordance with the usualwariable pitchairs'crew' practice pitch-changing means "are provided. Although thepitch-changing means may be arranged to operate either 'on themaster oron the'pro'ductionbla'des, in the em- 'rotating' rai rscrew disc shallnot tend to vibrate the 'ai'rscrew. This "iS- incident to the fact thatsprings24 and 25. The airscrew hub l5, in accordance with airscrewpractice, is preferably faired by the spinner 26; whereas the'remainderof the apparatus is faired by the cowling 21. The

electrical pickup and indicator device is of con-J ventionalconstruction being previously shown in other patents and for sale on theopen market." 1

Referring now to Figures 3 to 5, it will be seen that the reductiongear-box housing. l6 includes the driven gear wheel 28 fast on the shaft29 on which the airscrew hub I5 is splined. The master blade M ismounted in the hub in any convenient manner and preferably isessentiallycontrollable in pitch; whereas, as has been previously stated, theproduction blade P need not be mounted for pitch variation. r

In regard to the pitch-change apparatus, the

shafts 30 and 3| are normally held stationary,

the shaft 30 being keyed to the end plate 32; whereas the shaft 3| isoperated for rotation by the worm 33 engaging the worm wheel 34; the

- Worm 33 itself being driven through bevel gearing from the shaft l9,shown in Figures 1 and 2. The toothed annuli 35'and 3B normally. rotatewith the airscrew, thepinions 31 and 33 idling freely. Movement of the'worm 33 rotates the sun 39 which causes relative displacement of theannulus 35 'with respect to the airscrew hub through the medium of thepinions 31 the spindles of which are constrained to a uniform motionbythe pinions 38 acting in the annulus 36' on the sun 40. Therelative-"movement of the annulus 35 rotates the wheels 4| and 42mounted fixedly on the spindles and 44, having respectively 'rightandleft-hand screws working in 'the hub I5; and the arrangement istherefore such that for pitch-change operation'the spindle moves in thereverse sense, The spindles 43 and -44have adjustable end tappets' 45and 46 which bear on the projections, and 48, respectively,

extending from the root-mountingof the master blade M.

One method of mounting the blades is illustrated with reference'toFigures 6 and- 7.. As

' 43 is moved one way, whereas the spindle 44 shown, the airscrew driveshaftis provided with V projecting stubs 49, and the spider arm 50 isplaced on the stub in an initial position, which" may be approximatelythat shown in Figure 7. 'A difierential bolt 5| is then inserted throughthe h'ollowf spider arm and the bolt has an. upper 7 thread 52-and alower thread 53, which threads g respectively engage the internalthread54 of the spiderarm, and the internal thread 55 of the stub. Ahead 56 is provided on the difierential' bolt- 5| so that the latter maybe screwed in by *abox spanner. The threads 53 and 55 are steeper inpitch than the threads 52 and 54, with the result that when the threads.53 and 55 pick up on rotation of the differential bolt by means of abox-spanner operating in the slot 56, the differential action of thethreads forces the spider arm onto the stub until it reaches theposition shownin Figure 6.

It will be seen by reference to Figures 3 and 5 I that the airscrewblades are received directly on the spider arms 50, being held againstdisplacement radially outwards by the castellated end cap 51 threadedonto the barrel projections 58 surrounding the blade roots; and usualpacking and/or bearing means areprovided between the enlarged rootportion 59 and, the flange vofthe castellated end cap 51.

In operation of the apparatus, if the arrangement is such that thepitch-change mechanism operates on the master blade, the procedure is toset the'production blade by hand to the required nominal pitch, and thento run the apparatus up to its resonant speed. The pitch of the masterblade'is then adjusted until the minimum vibration is obtained, thusestablishing the difference in pitch between the master andthe'production blade that is necessary to achieve the best aerodynamicbalance possible in the circumstances. Alternatively, if thepitch-changing mechanism acts on the production blade, the procedure isto adjust the angle of the production blade, and then to run theairscrew up through the resonant range, noting the amount of unbalanceas indicated by the recording apparatus. The blade angle is adjustedagain and the airscrew run through the resonant range; the operationbeing repeated until the optimum setting is obtained. Forgiving a visualindictaion of the setting for the production blade which achieves the,minimum unbalance as compared with-the master blade,'it is convenient toprovide the Vernier fitting 60 on a forward'projection of the spindle 43in order t give a reading against the pointer 6 l.

Having found the amount and direction of pitch-adjustment required, theusual procedure to be followed is marking ofthe production blade in suchaway that by reference to a standard marking the blade can be mounted ina production airscrew hub in 'its' best condition of aerodynamicbalance. Thus, a plurality of blades which have been tested can bequickly and easily inserted into a production hub in such setting a thatthe airscrew will have the minimum degree of aerodynamic unbalance dueto' pitch errors which is possible in the circumstances.

Although in the foregoing description reference has beenmade to onemaster blade and one production blade, it will be realized that the hubmay be arranged to locate more than two blades. Thus; for instance wherea hub is intended for the reception of three blades, it is alwayspreferable to employ two carefully-matched master blades against which asingle production blade is to be tested. 1

It will be appreciated that the gyroscopic couple caused by the rackingmotion of the airscrew,

motor and frame on the supporting trunnions tends to' produceadditionalvibratingdisturbances which, under certain conditions, may reduoe oreven destroy the effectiveness of the apparatus as a means. fordetecting aerodynamic unbalance of the-airscrew blades. This actiondepends to a large extent upon the ratio between the moment of inertiaof the airscrew about a V diametral axis and themoment of inertia of'theairscrew motor and frame assembly about'the' trunnion axis, and can bereduced to negligible proportions by arranging that th'emoment of Vinertia of the airscrew, motorand frame about' the trunnion axis is alarge multipleof the mo-' ment of inertia of the airscrew'about adiametral axis. 'If required, additional mass can be added to the motoror frame for the purpose of making this ratio sufllciently large.

The apparatus may be arranged with its axis in any desired plane, butgenerally speaking it will be vertical or horizontal. The apparatus neednot necessarily be run at the same speed or with the same blade anglesas will be employed when the airscrew is in service, for it will beappreciated that the driving means in the balancetesting apparatusaccording to the present invention need not necessarily have the samepower output as an aircraft engine.

It may be possible in some cases to calibrate the apparatus so thatinstead of turning the blades until the balance is obtained, theindicating or recording means may be calibrated against master airscrewsor blades at various pitch settings, such calibrations being usable forestimating pitch errors of production blades.

What I claim is:

1. Apparatus for testing the aerodynamic bal ance of airscrew bladescomprising an airscrew hub to receive a production blade to beaerodynamically balanced against a master blade; means for driving theairscrew hub, the assembly of driving means and hub being mounted withfreedom to oscillate about an axis transverse to the axis of the hub;means for indicating the oscillations imparted to the said assemblyduring operation of the hub; and means for selectively adjusting thepitch setting of at least one of the blades during operation of the hub.

2. A construction as defined in claim 1, wherein the pitch-adjustingmeans comprises pitchchange gearing, elements driven by the gearing inrelatively reversed directions and means on the root mounting of one ofthe blades in contact with said elements to rotate the root mounting ofone of the blades in selective direction to effect a pitch change ofthat blade in the operation of the apparatus.

3. Apparatus for testing the aerodynamic balance of airscrew bladescomprising an airscrew hub to receive a production blade to beaerodynamically balanced against the master blade, means for driving theairscrew hub, the assembly of driving means and hub being mounted forfreedom to oscillate about an axis transverse to the axis of the hub,means for indicating the oscillations imparted to the said assemblyduring operation of the hub, gearing correlated to impart a pitchsetting adjustment of at least one of the blades, and members operatedby the gearing to impart rotative movement to the mounting of one of theblades during operation of the hub, whereby to adjust the pitch settingof that blade.

FRANK BERNARD HALFORD.

